1. Field of the Invention
The present invention generally relates to a signal transmission structure, in particular, the present invention relates to a multi-conducting through hole structure which can be used in a circuit board.
2. Description of Related Art
A conventional circuit board comprises multiple patterned circuit layers and a plurality of dielectric layers alternately laminated on top of each other. The patterned circuit layers are formed, for example, by patterning copper foils in a photolithographic process and an etching process. The dielectric layers are disposed between two neighboring patterned circuit layers to isolate the patterned circuit layers. In addition, the laminated circuit layers are electrically connected with each other through plated through holes (PTH) or micro vias.
The method of forming a micro via includes performing a photolithographic process or a laser drilling process to form a opening in the dielectric layer, and then filling the opening with a conductive material to form a micro via for electrically connecting at least two of the patterned circuit layers. The method of forming a PTH includes performing a mechanical drilling process to form a through hole in the laminated layers or a dielectric core layer, and then performing an electroplating process to form a metal layer on the inner surface of the through hole for electrically connecting two or more patterned circuit layers.
Since the conventional PTH occupies a larger area on a circuit board, the conventional PTH forms a limitation for raising the layout density of the circuit board. Therefore, a new type of multi-conducting PTH is developed. Compared with the conventional PTH having one conducting line passing through a single through hole, the new type of multi-conducting PTH has at least two conducting lines passing through a single through hole. Therefore, one new multi-conducting PTH is equal to a few of conventional PTHs, and the number of the conventional PTHs in the circuit board is decreased.
In order to form the new type of multi-conducting PTH in a circuit board, a laser cutting process can be added to the original processes of the conventional PTH. A metal layer on the inner surface of the through hole is cut by laser to form two or more segments or lines that vertically extending along the central axis of the through hole. Therefore, at least two metal lines are formed in the through hole, and the two pattern circuit layers located at the both ends of the lines formed of the metal layer respectively can electrically connected with each other by these metal lines.
There is still another method developed to form a multi-conducting PTH disclosed in application Ser. No. 11/065,496. The method can apply an additive process, a semi-additive process, or a negative process to form a plurality of metal lines (or strips) on the inner surface of the single through hole at the same time. The later method has higher speed and lower cost than those of the laser cutting process.
Since the layout density and the operating frequency of the circuit board are getting higher, the metal lines of the multi-conducting PTHs crowed inside the single through hole will raise heavy cross talk. Therefore, a proper arrangement of the multi-conducting PTH is very important.